Method and apparatus for measuring concentrations of gas mixtures

ABSTRACT

For identifying the concentration of individual types of molecules in a gas mixture, the gas mixture to be investigated is subjected to a selective pre-treatment and the product of this pre-treatment is then mass-spectrometrically investigated. For separate concentration identification of molecules having identical molecular mass, the gas mixture is ionized with primary ions that have an internal energy slightly above that required for generating product ions representing the respective type of molecule of interest and that have a pulse energy of such an amount that the kinetic effect on the ionization is negligible in comparison to the influence of the internal energy. The mass-spectrometric investigation then supplies the sought concentration of the individual type of molecule in a simple and precise fashion.

The invention is directed to a method for identifying the concentrationsof individual types of molecules in gas mixtures, whereby the gasmixture to be examined is subjected to a selective pre-treatment and theproduct of this pre-treatment is then examined mass spectrometrically.The invention is also directed to an apparatus for the implementation ofsuch a method.

Deriving as a direct problem in the mass-spectrometric examination ofgas mixtures containing different types of molecule for theconcentrations of individual types of molecule, is that differentmolecules having identical molecular mass cannot be separatelyrecognized by the mass spectrometer due to the physical conditions onwhich the function thereof is based. For example, this applies to thecombination of nitrogen monoxide (NO) and formaldehyde (CH₂ O) eachhaving 30 amu (atomic mass unit), to the combination of oxygen (O₂) andmethylalcohol (CH₃ OH) each having 32 amu, and to the combination of COwith N₂ and C₂ H₆ each of which has 28 amu.

Since measurement with a mass spectrometer is relatively simple tomanipulate, and is precise and fast, efforts were undertaken to remedythis disadvantage. Methods and, respectively, apparatus of the speciesinitially cited have been disclosed wherein the gas mixture to beexamined is subjected to a selective pre-treatment in order to enable apre-separation of the types of molecules of interest that have the samemolecular mass.

Among the apparatus that have become known are those sold under thedesignation GCMS Systems. In the GCMS Systems apparatus a gaschromatograph is employed thereto, this enabling a chronologicallygraduated feed of the individual types of molecules to the massspectrometer that carries out the actual measurement. The chronologicalgraduated feed occurs as a result of the different transit times of theindividual types of molecules and thus enables a separateidentification--at least within limits--of the individual types ofmolecules or, respectively, of their concentration. For example, themolecules N₂ O and CO₂ that have the same molecular mass can thus beseparately acquired, these having noticeably different transit times inthe gas chromatograph.

In this context, however, there are already problems. For example, inthe detection of different alcohols that have similar transit times inthe gas chromatograph and that are fragmented in the mass spectrometer,the separate identification of individual types of molecules isimpossible in practice. The extremely low response speed of these GCMSSystems is to be cited as a further disadvantage, this response speed,of course, being greatly restricted by the transit times of the gasmixtures being examined or, respectively, of the types of molecules tobe detected in the gas chromatograph.

SUMMARY OF THE INVENTION

An object of the present invention is an improved method or,respectively, apparatus of the species initially cited such that thedisadvantages of the known arrangements are avoided and such that, inparticular, the concentration of individual types of molecules havingthe same molecular mass can be identified in a simple way in gasmixtures under examination without great delays caused by themeasurement or by the preparation of the gas mixture for themeasurement.

For separate identification of the concentration of molecules having thesame molecular mass, this is inventively achieved in that the gasmixture to be examined is ionized with primary ions under single pulseconditions in hard vacuum, whereby the primary ions employed have aninternal energy slightly above, preferably ≦2 eV above, that requiredfor generating product ions representing the respective type of moleculeof interest, and have a translational pulse energy of such an amount inthe respective center-of-mass system that the kinetic effect on theionization is negligible in comparison to the influence of the internalenergy, preferably a pulse energy below 10 eV, and in that themass-spectrometric examination is undertaken at the product ions.

In accordance with the invention, an apparatus for identifying theconcentrations of individual types of molecule in gas mixtures includesan arrangement for selective pre-treatment of the gas mixture to beexamined and a mass spectrometer arrangement for the examination of theproduct of this pre-treatment; wherein the pre-treatment arrangementcomprises an ion source for primary ions, a reaction space connecting tothe ion source and having a feed for the gas mixture to be examined, anda pump for creating hard vacuum; in that the mass spectrometerarrangement is arranged at that side of the reaction space thatessentially faces away from the ion source; in that the ion source emitsprimary ions having an internal energy slightly above preferably ≦2 eVabove that required for generating product ions representing therespective type of molecule of interest and with a translational pulseenergy below 10 eV; and in that the reaction space is surrounded by anoctopole arrangement at which a radio frequency guide potential isadjacent.

In accordance with the invention, thus, the selective pretreatment ofthe gas mixture to be examined is directly undertaken by the ionizationwith the primary ions having a defined internal energy (ionizationpotential) given limited translational pulse energy, the stateddisadvantages of the GCMS Systems initially set forth being therewithavoided and the concentrations of molecules having the same molecularmass also being separately identifiable in a simple and fast way.

As they are presently usually operated, mass spectrometer arrangementshave significant deficiencies as a consequence of the ionization processwith electrons that is employed, these deficiencies greatly restrictingtheir applicability to various problems or, respectively, making thisimpossible. Particularly included among these deficiencies are thefragmentation from various substances, the identical fragmentation fromvarious substances, as well as the interferences due to identical massof the ions from various substances. In the ionization event withelectrons, these are usually brought to a kinetic energy of at least 70eV for this process. The activation cross sections or, respectively, theionization efficiency lies on usable orders of magnitude given theseenergies; however, the fragmentation of molecules into fragments must beaccepted as a critical disadvantage. Thus, for example, CO₂ +e⁻ becomesCO₂ ⁺, O₂ ⁺, CO⁺, O⁺ and C⁺. CO+e⁻ becomes CO⁺, C⁺ and O⁺. O₂ +e⁻becomes O₂ ⁺, O⁺. It can be easily seen that the identification of theconcentration of individual molecule types in gas mixtures thus becomesimpossible in most cases.

Let the following serve as an example of the fragmentation of moleculeshaving similar structure in the atomic structure in similar spectra: CH₃OH+e⁻ becomes C⁺ ; CH₃ ⁺, CH₂ ⁺, CH⁺ ; OH⁺, O⁺ ; CH₃ OH⁺. C₂ H₅ OH+e⁻becomes C⁺ ; CH₃ ⁺, CH₂ ⁺, CH⁺ ; OH⁺, O⁺ ; CH₃ OH⁺ ; C₂ H₅ OH⁺.

The present invention then proceeds on the basis of the perception thatthe neutral molecules can be converted into ions without thereby beingionized into fragments due to the selection of a type of primary ionwhose ionization potential (inner potential) lies only insignificantlyabove that of the species to be ionized. For example, CH₃ OH+Xe⁺ onlybecomes CH₃ OH⁺ +Xe or, respectively, C₂ H₅ OH+Xe⁺ only becomes C₂ H₅OH⁺ +Xe. As a consequence of the selection of the ionization potentialof the primary ions in accordance with the present invention, theseparate identification of molecule concentrations of identical moleculemasses is enabled in an extremely simple way by means of selectiveionization.

For separate identification of hydrocarbon compounds, it is provided,according to a further development of the method of the invention, thatthe primary ions employed exhibit an internal energy slightly above,preferably ≦2 eV above, that required for generating a product ion uponsimultaneous abstraction. The multitude of possibilities of makingmolecules of identical molecular mass distinguishable by the selectiveionization process in the subsequent mass-spectrometric examination isexpanded by this ionization with H-abstraction.

The following should be noted here regarding the technical term"H-abstraction":

In exothermic reactions of positively charged particles with molecules,specifically hydrocarbon compounds, the excess energy of the reaction(difference between the ionization potentials) is distributed in thefollowing way:

(1) into translational energy of the reaction partners given a low ΔE(<0.1 eV) excess;

(2) into the molecules given ΔE<1 eV oscillatory impulse of the bonds;and

(3) given ΔE≈1 eV<3 eV dissociation, i.e., separation of a bond (atom)with the lowest bonding energy, from the atomic bond.tbd.molecule.

In hydrocarbon compounds, this is an H-atom ("H-abstraction"). Inchloronated hydrocarbons, generally halogenated hydrocarbons, this isthe halogen atom.

For the separate identification of metallo-organic compounds, theprimary ions used, according to a further development of the invention,have an internal energy in the range from 4.5 to 6.8 eV. Theprobabilities of possible interferences with hydrocarbon compoundshaving the same molecular masses become greater and greater in theidentification of metallo-organic compounds, so that the identificationof the molecules on the basis of the characteristic isotopedistributions becomes increasingly more difficult with the number ofatoms in the molecule. The metallo-organic compounds whose ionizationenergies lie in this range are selectively ionized by the selection ofthe primary ions employed, this without simultaneous ionization of thehydrocarbon compounds, so that a simple mass-spectrometric detectionbecomes possible.

According to further developments of the invention, for separateidentification of oxygen and methylalcohol; CO₂, CO, and O₂ ; ornitrogen monoxide and formaldehyde, krypton⁺ ions can be used as primaryions, these best satisfying the said demands made of the internalenergy.

According to another, advantageous development of the invention, forseparate identification of CH₃ OH and C₂ H₅ OH or, respectively, ofdi-nitrogen monoxide and carbon dioxide, xenon⁺ ions can be used asprimary ions.

BRIEF DESCRIPTION OF THE DRAWING

The only FIGURE is a cross-sectional view of an apparatus embodyingprinciples of the invention having an ion beam emitter and amass-spectrometer arranged at opposite ends of a chamber, wherein a gasmixture to be analyzed is introduced.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The method of the invention shall now be set forth in greater detailwith reference to the following examples.

EXAMPLE 1

Separation of CO, N₂, C₂ H₆ by selective ionization with Kr⁺ ions (theeV value in parenthesis denotes the internal energy or, respectively,the ionization potential)

    Kr.sup.+ (.sup.2 P.sub.3/2) (14 eV)+N.sub.2 →N.sub.2.sup.+ (15.5 eV)+Kr;

ΔE=-1.5 eV=endothermic charge exchange; ionization impossible. ##STR1##ΔE=+0.1 eV=exothermic charge exchange, ionization at 28 amu; takingenthalpies of formation ΔH_(f) in kcal into consideration (1 eV 23.06kcal). ##STR2## ΔE=31.8 kcal=1.4 eV exothermic; ionization withH-abstraction at 27 amu; (for Kr⁺ +C₂ H₆ →C2H₆ ⁺ +Kr, ΔE would be high;the above reaction therefore ensues with considerably greaterprobability).

EXAMPLE 2

Separation of oxygen O₂ and methylalcohol CH₃ OH with selectiveionization by Kr⁺ ions. ##STR3## ΔE=1.9 eV exothermic; ionization at 32amu. ##STR4## ΔE=+6 kcal exothermic; ionization with H-abstraction at 31amu.

EXAMPLE 3

Separation of nitrogen monoxide NO (30 amu) and formaldehyde CH₂ O)=(30amu). ##STR5## ΔE=+4.8 eV exothermic; ionization at 30 amu or →NO⁺ (aΣ⁺)(14.2) . . . ΔE≃O (energy-resonant). ##STR6## ΔE=+22 kcal exothermic;ionization with H-abstraction at 29 amu.

EXAMPLE 4

Separation of di-nitrogen monoxide N₂ O (44 amu) and CO₂ (carbondioxide) (44 amu) by selective ionization with Xe⁺ ions. ##STR7## ΔE=0.5eV; ionization at 44 amu.

    Xe.sup.+ (13.4 eV)+CO.sub.2 →CO.sub.2.sup.+ (13.7)+Xe;

ΔE=-0.3 eV exothermic; no ionization possible.

The method of the invention thus makes a fast analysis (real-timeanalysis) of gas mixtures possible, i.e. makes an identification of theindividual molecular components in gas mixtures possible, this being ofutmost interest for many applications in industry and research such as,for example: the study of rapidly occurring chemical reactions forclarification of the reaction kinetics; and the measurement ofbrief-duration, metastable intermediate products and radicals inchemical conversions or, respectively, catalytically functioning agents.

All applications of this type can be realized in accordance with thepresent invention wherein it is essentially provided that the ionizationneeded for the employment of the mass spectrometer, i.e. the conversionof all neutral molecules to be investigated into ions, is carried outwith a positively or negatively charged, intense ion beam having aprecisely defined, internal energy that enters into interaction with themolecules to be ionized. By selecting a type of primary ion whoseinternal energy lies only insignificantly higher than the ionizationpotential of the species to be ionized, the molecules are converted intoions without being ionized into fragments. The corresponding,translational pulse energy between the primary ions and the gasparticles must thereby be kept so low that the kinetic effects on theionization are small in comparison to the influence of the internalenergy.

The apparatus of the invention for the identification of theconcentration of individual types of molecule in gas mixtures shall beset forth in greater detail below with reference to the schematicdrawing.

The apparatus comprises an ion source 1 of an arbitrary type of nofarther interest here whose critical specification in the presentcontext is such that the primary ions that are generated and that emergefrom the ion source in the direction z have an internal energy slightlyabove, preferably ≦2 eV above that required for generating product ionsrepresenting the respective molecule type of interest and have atranslational pulse energy of such a size in the respectivecenter-of-mass system of primary ion/molecule that the kinetic effect onthe ionization is negligible in comparison to the influence of theinternal energy, preferably having a translational pulse energy below 10eV. For example, a closed electron pulse ion source can thereby beemployed, a primary gas A flowing therethrough. This primary gas A isionized by electrons in processes e⁻ +A→A⁺ +2 e⁻, wherewith primary ionsA⁺ are available.

A reaction space 2 joining to the ion source 1 is also provided, as is amass spectrometer arrangement 3--preferably a quadrupole massspectrometer--that joins the reaction space 2 in the principal movingdirection z of the primary ions supplied by the ion source 1. Thereaction space 2 to which the gas mixture to be examined can be suppliedvia a connector 4 is surrounded by octopole arrangement 5 having a radiofrequency 8-pole field acting perpendicularly relative to the principalmoving direction z of the primary ions, this radio frequency 8-polefield serving the purpose of collecting, holding or, respectively,guiding the primary ions that exhibit an extremely low kinetic energy.The product ions are selected in the mass spectrometer arrangement 3 or,respectively, in the corresponding quadrupole in a known way that is ofno further interest here, and proceed to an ion sensor 6 that canlikewise be of a known or, respectively, standard structure and whosefunction and action are of no further interest here.

A pump for hard vacuum evacuation of the reaction space 2 is referenced10; various other auxiliary devices such as diaphragms or, respectively,lenses for the ion beam or further vacuum pumps and seals and the likethat are partly indispensable for the function of the arrangement instandard operation are not shown here since they are of secondarysignificance in the context of the present invention.

In longitudinal direction--i.e. in the principal moving direction z ofthe primary ions emerging from the ion source 1 in a ray--, the octopolearrangement 5 is divided into three individual regions 7, 8, 9 that areelectrically insulated from one another which--as seen from the ionsource 1 to the mass spectrometer arrangement 3--lie at a respectivelyincreasing negative dc potential in comparison to the preceding region.In addition to the radial guidance field produced by the radio frequencypotential that holds the ions in the inside of the octopole arrangement5, an E-field is produced in axial direction in this way, this seeing toa rapid and complete removal of the product ions in the direction towardthe mass spectrometer and, thus, enabling a rapid analysis with highprecision.

We claim:
 1. An apparatus for identifying concentrations of individualtypes of molecules in a gas mixture, comprising:a pre-treatmentarrangement for selective pre-treatment of the gas mixture includinganion source for producing primary ions having an internal energy slightlyabove that required for generating product ions representing a type ofmolecule of interest and with a translation pulse energy below 10 eV, achamber coupled to said ion source, a feed line in communication with aninterior of said chamber for introduction therein of the gas mixture, apump operatively coupled to the chamber and in communication with theinterior thereof for creating a vacuum therein, and an octopolearrangement arranged within the chamber at which a radio frequency guidepotential is adjacent; and a mass-spectrometer arrangement incommunication with a discharge end of said chamber so that product ionsemitted from the chamber are mass spectrometrically examined.
 2. Themethod of claim 1 wherein the primary ions employed have an internalenergy ≦2 eV above that required for generating product ions.
 3. Themethod of claim 1, wherein the primary ions employed have atranslational pulse energy less than about 10 eV.
 4. The method of claim1, wherein the primary ions employed have an internal energy <2 eV abovethat required for generating product ions and a translational pulseenergy less than about 10 eV.
 5. The method of claim 1, for separateidentification of hydrocarbon compounds, the primary ions employed havean internal energy slightly above that required for generating a production given simultaneous H-abstraction.
 6. The method of claim 5, whereinthe primary ions employed have an internal energy less than about 2 eVabove that required for generating a product ion given simultaneousH-abstraction.
 7. The method of claim 5, wherein, for separateidentification of metallo-organic compounds, the primary ions employedhave an internal energy in the range from about 4.5 to about 6.8 eV. 8.The method of claim 5, wherein, for separate identification of oxygen(O₂) and methylalcohol (CH₃ OH), krypton⁺ (Kr) ions are used as primaryions.
 9. The method of claim 5, wherein, for separate identification ofCO₂, CO, O₂, krypton⁺ (Kr) ions are employed as primary ions.
 10. Themethod according to claim 5, wherein, for separate identification of CH₃OH and C₂ H₅ OH, xenon⁺ (Xe) ions are employed as primary ions.
 11. Themethod of claim 5, wherein, for separate identification of nitrogenmonoxide (NO) and formaldehyde (CH₂ O), krypton⁺ (Kr) ions are employedas the primary ions.
 12. The method of claim 1, wherein, for separateidentification of di-nitrogen monoxide (N₂ O) and carbon dioxide (CO₂),xenon⁺ (Xe) ions are employed as the primary ions.
 13. An apparatus foridentifying concentrations of individual types of molecules in a gasmixture, comprising:a pre-treatment arrangement for selectivepre-treatment of the gas mixture includingan ion source for producingprimary ions, a chamber coupled to said ion source, a feed line incommunication with an interior of said chamber for introduction thereinof the gas mixture, a pump operatively coupled to the chamber and incommunication with the interior thereof for creating a vacuum therein,and an octopole arrangement arranged within the chamber at which a radiofrequency guide potential is adjacent; and a mass-spectrometerarrangement in communication with a discharge end of said chamber sothat primary ions emitted from the ion source having an internal energyslightly above that required for generating product ions representing atype of molecule of interest and with a translation pulse energy below10 eV are mass spectrometrically examined.
 14. The apparatus of claim13, wherein the primary ions produced have an internal energy less thanor equal to about 2 eV above that required for generating product ionsrepresenting the type of molecule of interest.